Resistance welding is now widely used in most applications requiring the joining of metals, such as the steel used in the manufacturing of automobiles. With the advent of the microprocessor, weld controllers have become more sophisticated and use a variety of control techniques to ensure the quality of welds throughout the life of the contact tips as they wear out. Regardless of the process or control technique used, most weld controllers consist of several basic components. These include a weld control module, a power module, a weld transformer and the contact tips. The power module usually consists of power semiconductors such as silicon controlled rectifiers (SCRs) that switch incoming power to the weld transformer according to a preset weld program as generated by the control module. The weld transformer will transform the incoming power to a high current pulse that is coupled to the contact tips to create a weld to a workpiece that is between the contact tips.
A weld program will use phase angle control to switch the power modules. In order to maintain the desired level of heat delivered to the weld, the proper phase angle to fire the SCRs will be a function of the condition of the power source delivering power to the weld control and subsequently through the weld control to the weld transformer.
An early type of a voltage compensated welder control is disclosed in U.S. Pat. No. 4,289,948 which describes an approach for developing timing signals based on measuring the line voltage and determining its nominal value. This nominal value is compared with an expected or desirable voltage level. The difference is used in an empirically developed equation to determine a new firing angle which will be necessary to raise or lower the effective voltage applied to the weld transformer and the contact tips in order to keep the welding current constant and independent of line voltage variations. Under the assumption that the load magnitude and power factor are accurately known, and that an infinitely stiff source of voltage appears at the input to the weld controller, firing the SCRs will keep the weld current constant. However, since load impedance varies from part to part and the signature of the contact tips change due to wear, the actual line voltage is rarely the nominal design voltage.
In general, the actual line voltage is a function not only of the source voltage but also of line impedance. The line voltage can differ from the nominal designed voltage since the voltage source is a real voltage source generated by a power company and subject to a power distribution system and hence may not be at the nominal design voltage of the weld control. The presence of line impedance causes a voltage drop proportional to the current flowing into the weld. Comparing the actual weld current to a targeted load current, it is proportional to the ratio of the actual line voltage to the nominal line voltage and it is reduced by a factor that is a function of the ratio of the nominal load current to the total available nominal short circuit current of the weld.
The prior art assumes that the voltage source is stiff with no line impedance. The premise is that the weld controller is capable of measuring the line voltage of the weld controller from a present weld pulse and can correct the next weld pulse based on this measurement. In the steady state, this provides the desired result. However, there is a transient response associated with this approach which limits its effectiveness. In the steady state, the actual weld load current will be the nominal load current. However, the first two cycles of weld current are significantly lower than the nominal desired current for power distribution systems that are very stiff since the initial load current does not deviate significantly from the nominal desired current. The prior art, while providing excellent steady state response, has a transient characteristic which is undesirable in welding applications, particularly where a small number of cycles are used to create the weld, such as in aluminum welding. A weld pulse comprising only 3-6 cycles of relatively high current is very common in modern applications, so an improvement in the transient response of the weld control would be most beneficial.
It would be desirable to develop a system or method whereby this transient characteristic is reduced and the effect of line impedance can be compensated, resulting in a marked improvement in the transient performance of the the present weld controller over prior art controls.